The prevalence of obesity and associated co-morbidities, including type 2 diabetes and cardiovascular disease (CVD), has increased dramatically in the past several decades. While weight loss is the ideal approach to reduce the negative metabolic consequences of obesity, it is clear that sustained weight loss is difficult to achieve. In fact, only 20% of people who lose at least 10% of their body weight are able to maintain that loss for greater than 2 years. These bouts of weight loss followed by subsequent weight gain lead to ?weight-cycling?. Interestingly, several studies in humans demonstrate that weight-cycling increases the risk of developing metabolic diseases. While the potentially deleterious effects of weight-cycling are recognized, the mechanisms by which weight- cycling increases metabolic dysfunction remain unknown. During the past decade, we have come to understand that the immune system plays a key role in the pathological consequences of obesity. Metabolic organs such as the liver, muscle, and adipose tissue (AT) accumulate immune cells that subsequently impact the insulin sensitivity of the parenchymal cells. In particular, obesity results in a dramatic increase in the number of inflammatory AT macrophages and AT T lymphocytes (ATTs). Interestingly, the accumulation of T cells in obese AT appears to be antigen-driven and is also characterized by the formation of memory cells. To determine if weight cycling alters immune responses in AT, we developed a mouse model of weight cycling using alternating high fat (HF) and low fat (LF) diet feeding. Similar to what is seen in humans; the weight-cycled mice had increased fasting glucose levels and impaired systemic glucose tolerance compared to mice that gained weight but did not cycle (weight-gain controls). Furthermore, AT-specific insulin signaling was abolished in the weight-cycled mice. At the end of the study, the macrophage populations in AT were unchanged in their number and phenotype. However, ATT number and the expression of multiple TH1-associated genes were significantly increased in the AT of the weight-cycled mice. These data demonstrate that weight cycling induces a potent T cell-driven adaptive immune response in the AT and suggest that weight cycling actually induce a secondary adaptive immune response. Thus, the overall hypothesis of this application is: weight-cycling results in an accelerated secondary adaptive immune response that heightens inflammation in AT, leading to local and systemic insulin resistance. This hypothesis will be tested in the following 3 aims: 1) To determine whether weight cycling alters ATT phenotype and function; 2) To determine if weight cycling induces secondary immune responses in AT; 3) To determine whether weight cycling modulates regulatory T cell (Treg) phenotype and function.